Cobalt Fischer-Tropsch catalysts having improved selectivity

ABSTRACT

The promoter(s) Mn oxide or Mn oxide and Zr oxide are added to a cobalt Fischer-Tropsch catalyst combined with the molecular sieve TC-103 or TC-123 such that the resultant catalyst demonstrates improved product selectivity, stability and catalyst life. The improved selectivity is evidenced by lower methane production, higher C5+ yield and increased olefin production.

STATEMENT

The Government of the United States of America has rights to thisinvention pursuant to Contract No. DE-AC22-84PC70028 awarded by the U.S.Department of Energy.

FIELD OF THE INVENTION

The present application relates to the field of cobalt Fischer-Tropschcatalysts in combination with various molecular sieves which have beenfurther promoted to improve product selectivity and stability.

BACKGROUND OF THE INVENTION

Iron Fischer-Tropsch (F-T) catalysts have generally been preferredcommercially over cobalt based catalysts and are presently the onlycommercial F-T catalyst used.

While cobalt catalysts have the benefit of higher activity and betterselectivity to motor fuels they suffer from their inherent production ofexcess methane (an undesirable product) as well as the paraffinic natureof the product. It would be an important catalyst improvement if astable cobalt F-T catalyst was discovered which demonstrated reducedmethane production, increased C5+ yield and improved olefin content,especially in the C5- range.

Prior art teaching for the use of promoted cobalt oxide in the FischerTropsch process has been well established, including the commercial useof a thorium, magnesium promoted cobalt kieselguhr catalyst in Germanyin the 1930's and 40's. A large number of literature studies have beenpublished and patents issued in the area, many dealing with the use ofvarious promoters to improve catalyst performance.

An excellent review of past publications on cobalt Fischer-Tropschcatalyst was reported by R. B. Anderson, "The Fischer-TropschSynthesis", Academic Press, Orlando FL, 1984. In the review are lists ofpromoters and catalysts studied in the past, included are sightings ofthe use of Mn and Zr promoters. Listed below is a summary of theinformation presented in this article related to the use of the Mn andZr promoters. Fischer and Koch showed Mn added to a cobalt kieselguhrcatalyst was effective at shifting the product distribution towardheavier product as was also observed for the catalysts in thisinvention. Work by Eidus and Bulanova showed a similar effect on theaddition of ZrO2 to the same type of catalyst, this was not observedupon our addition of ZrO2 to the Mn promoted Co/TC-123 catalyst system.No work to our knowledge has been reported on the use of a combined Mnand Zr promoted catalyst

Dent, A. L. and Lin, M., Adv. Chem. Ser. 178,47 (1979) reported that theaddition of Mn to a cobalt alumina catalyst increased the olefin contentin the product, which is consistent with our data.

The only prior art of which the applicants are aware relating Zr forimproving the stability of a cobalt F-T catalyst was reported by Eidusand Bulanova U.S.S.R. 150,102, Sept. 26, 1962, Appl. Nov. 27, 1954. Inthis work Zr was used in place of thorium to reduce the sensitivity ofthe catalyst to super heating. The applicants are not aware of any priorart which teaches the use of Mn to improve cobalt catalyst stability.

SUMMARY OF THE INVENTION

The present invention is directed to a cobalt Fischer-TropschCatalyst/Molecular Sieve combination which has been modified using apromoter selected from the group comprising Mn oxide or the combinationof Mn oxide and Zr oxide.

It has been found that the resulted catalyst demonstrates superiorproduct selectivity and stability which is evidenced by lower methaneproduction, higher C5+ yields, increased olefin production, and longercatalyst life.

DESCRIPTION OF THE INVENTION

We have found that when the promoter(s) Mn oxide or the combination ofMn oxide and Zr oxide are added to a cobalt F-T catalyst supported onthe molecular sieves TC-103 or TC-123 that the catalyst s performance issignificantly improved. The use of a cobalt catalyst in combination withTC-103 (acid extracted LZ-10) was previously reported by Rabo et al. -U.S. Pat. No. 4,652,538, where TC- 103 represents an acid extractedultrahydrophobic Y molecular sieve. The use of the molecular sieveTC-123 is disclosed by Miller et al (U.S. patent application Ser. No.72,748, filed concurrently herewith) where TC-123 represents a steamtreated, acid extracted LZ-210. Addition of Mn oxide was found tosignificantly reduce methane production. This decrease resulted fromshifting of the product slate toward heavier, more desirable product. Asignificant increase in product yield above the motor fuels range wasobserved, but this could easily be hydroprocessed back into the motorfuel boiling point range. The Mn oxide promoter acts to reduce thehydrogenation ability of the cobalt catalyst, promoting hydrocarbonchain growth over chain termination. Consistent with this theory theolefin content of the F-T product is increased in the presence of the MNadditive.

Surprisingly, in spite of the reduced hydrogenation activity, theaddition of the Mn promoter was also found to greatly increase thecobalt catalyst's stability. A ten fold decrease in the deactivationrate was observed upon addition of Mn to a cobalt TC-123 catalyst.Comparison of a Mn promoted γ-alumina catalyst to that of a TC-103catalyst showed that a synergistic stabilizing effect existed betweenthe Mn promoter and the molecular sieve chosen, not observed for atypical catalyst support like γ-alumina. Further stabilization of themolecular sieve supported Mn promoted cobalt catalyst was found by theaddition of Zr oxide, without significantly effecting the productselectivity.

Characterization of the cobalt molecular sieve catalysts shows thepromoted cobalt being encapsulated within the secondary pore structureof the molecular sieve support. The secondary pores in the molecularsieve crystal aid in tailoring and maintaining the cobalt particles atthe optimum size for maximum activity and selectivity while allowing thesyngas and products to diffuse through the primary zeolite porestructure. The promoters further prevent the cobalt metal particle fromsintering and moving to the outside of the molecular sieve crystals.

EXAMPLES

While the invention has been described above the details of the presentinvention will be better understood by recourse of the followingexamples which serve to illustrate the following:

The addition of Mn oxide to a cobalt TC-123 catalyst significantlyimproves the catalyst's stability, product selectivity and activity.

The addition of Mn oxide to a cobalt/TC-103 catalyst significantlyimproves the catalyst's stability and product selectivity.

The addition of Zr oxide to a Mn promoted Co/TC-123 catalyst furtherincreased catalyst stability while having little effect on productselectivity.

The Mn promoted cobalt TC-103 catalyst has superior stability to a Mnpromoted cobalt supported on a conventional support like γ-alumina.

EXAMPLE I

The following example compares a manganese oxide promoted and anon-promoted cobalt/TC-123 F-T catalyst. The Mn promoter was found tosignificantly improve the activity, product selectivity and stability ofthe cobalt/TC-123 catalyst.

The catalysts were both prepared by the same procedure. Typically themolecular sieve TC-123 (100q anhydrous) was pore filled with an ethyleneglycol solution containing 55.8 g Co(NO3) 6H20, 7.21 Mn(NO3)2 xH2O and48.0 g ethylene glycol. The TC-123 molecular sieve support was preparedby treating steam stabilized, ammonium exchanged LZ-210 with a SiO₂ /Al₂O₃ ratio of 9.0 with 1 atmosphere of steam at 750° C. for 1 hour,followed by acid extraction in 3M HCl under reflux conditions for 3hours. The support was filtered, water washed and dried at 11° C.overnight. Prior to pore filling the ethylene glycol solution was heatedto 50° C. for 1 hour. The powder was dried using the followingprocedure: 110° C. for 10 hours, 200° C. for 30 minutes, and 450° C. for4 hours. The catalyst was then bonded with 15% silica (Nalco), extrudedinto 1/8" extrudates, dried at 110° C. and calcined at 250° C. for 2hours. The calculated percent of cobalt and manganese in the catalystsbased on raw materials used were 8.3% Co, 0.0% Mn, and 8.2% Co, 1.6% Mn.

Catalyst samples were loaded into a Berty type internal recyclingreactor and hydrogen treated at 350C, 300 psig for 18 hours and exposedto 1:1 H2:CO syngas at 220° C. The catalysts were tested at 240° C., 300psig, 300 GHSV, 1:1 H2:CO as well as 260° C., 500 psig, 300 GHSV, 1.5:1H2:CO.

Results of the testing are illustrated in Table 1 and 2. The Mn oxidepromoted catalyst performed significantly better than the non-promotedcatalyst under both the 240° C. and 260° C. conditions. The syngasconversion activity of the cobalt catalyst was improved by more than10%. Furthermore, selectivity of the promoted catalyst showed adesirable decrease in methane production, about 1/2 that of thenon-promoted catalyst. In addition, a significant increase in C5+ yieldwas also observed. The majority of the C5+ yield increase was observedin the 650F+wax fraction which is easily hydroprocessed into the motorfuels range. Hydrogen economy and product quality was also improved bythe increase of olefin content of the product, evidenced by the increasein the olefin/paraffin ratio of the C4 product.

In addition to the benefits cited above a major benefit of the Mn oxidepromoter was it's effect on catalyst stability. The catalyst showed aten fold decrease in the percent syngas conversion loss per hour thanthe non-promoted catalyst under the 260° C. conditions. The deactivationrate was based on a least squares analysis. Such reduced deactivationrate implies a several fold increase in catalyst life. Since catalystcost is an important component of F-T product cost, such a largeincrease in catalyst life provides important economic benefit for the Mnpromoted catalyst.

                  TABLE 1                                                         ______________________________________                                        Comparison of Co/TC-123 and Mn Oxide Promoted Co/                             TC-123 Fischer-Tropsch Catalyst                                               Catalyst:   Co/TC-123      Co/Mn/Tc-123                                       ______________________________________                                        Temperature 240° C.                                                                        260° C.                                                                           240° C.                                                                      260° C.                           H2:CO       1:1     1.5:1      1:1   1.5:1                                    Pressure    300     500        300   500                                      Conversion  42      73         48    81                                       CH4         6.7     26.0       3.3   12.2                                     C2-C4       9.2     14.3       7.8   10.4                                     C5-350° F.                                                                         25.0    26.2       21.4  29.6                                     350-650° F.                                                                        35.1    22.3       29.0  30.1                                     650° F.+                                                                           24.6    11.2       38.6  17.7                                     C5+         84.8    59.7       88.9  77.4                                     C4 ole./par.                                                                              1.8     .5         3.7   .9                                       Stability.sup.(a)                                                                         --      .2         --    .02                                      ______________________________________                                         Space velocity  300 GHSV                                                      .sup.(a) Percent loss in syngas conversion per hour.                     

EXAMPLE II

The following example compares a manganese oxide promoted cobalt oxidecatalyst and a non-promoted cobalt oxide catalyst both supported onTC-103 (acid extracted LZ-10). Similar to Example I, the Mn promoter wasfound to improve catalyst performance by increasing catalyst stabilityand improving product selectivity.

The two catalysts were prepared under almost identical conditions. Thenon-promoted catalyst utilized the procedure described for the catalystsin Example I. The Mn promoted catalyst was prepared by a similar method,however the drying and calcining procedure (ramping to 450° C.) was notperformed and the pore filled material was dried at 110° C. overnight.The calcining procedure change should have little effect on the overallcatalyst performance. The calculated percent cobalt and manganese in thecatalysts based on raw material used were 12.8% Co, 0.0% Mn and 12.3%Co, 2.4% Mn. Fischer-Tropsch testing was performed as described inExample I. The non-promoted catalyst was exposed to 1:1 H2:CO and 260°C. initially and the Mn promoted catalyst was exposed to 1:1 H2:COinitially at 220° C.

Results of the testing are illustrated in Table 2.

Similar to the TC-123 system (Example I) the Mn oxide promoter served todecrease methane production, increase C5+yield and increase the olefinto paraffin ratio of the product. Unlike the TC-123 system the promotedcatalyst showed lower syngas conversion activity however it demonstratedsuperior stability both in syngas conversion and in product selectivity.The syngas conversion of the non-promoted catalyst showed a rapid drop,about 9% conversion in 94.5 hours compared to only 1.8% over 144 hoursfor the promoted catalyst. Thus, under practical conditions over desiredcatalyst life times of greater than 6 months the Mn promoted catalyst isexpected to show much superior overall activity relative to thenon-promoted catalyst.

                  TABLE 2                                                         ______________________________________                                        Comparison of Co/TC-103 and Mn Oxide Promoted Co/                             TC-103 Fischer-Tropsch Catalysts                                              Catalyst:   Co/TC-103      Co/Mn/TC-103                                       ______________________________________                                        TOS  71.1           165.5      167.5 311.5                                    H2:CO                                                                              1:1            1:1        1:1   1:1                                      Pressure                                                                           300            300        300   300                                      Conversion                                                                         77.8           68.5       54.0  52.2                                     CH4  10.1           16.1       6.5   6.3                                      C2-C4                                                                              9.6            10.9       10.7  11.1                                     C5-420° F.                                                                  39.4           39.9       34.4  36.0                                     420-700° F.                                                                 31.0           25.1       28.5  29.4                                     700° F.+                                                                    9.9            8.1        19.9  17.3                                     C5+  80.3           73.1       82.8  82.6                                     C4 ole./par.                                                                       1.6            .9         2.2   2.2                                      ______________________________________                                         Space velocity = 300 GHSV, Temperature = 260° C.                  

EXAMPLE III

The following example compares a Mn oxide promoted Co/TC-123 catalyst toone promoted by both Mn oxide and Zr oxide. The added zirconium oxidepromoter greatly increased catalyst stability while having only minoraffects on the catalyst's activity and selectivity.

The catalysts were prepared as described in Example I except in the Mnand Zr promoted catalysts ZrO(NO3)2 (5.62 g) was added to the ethyleneglycol solution used to pore fill the TC-123 molecular sieve. Thecalculated percentage of cobalt, manganese, and zirconium in thecatalysts based on raw materials used were 8.2% Co, 1.6% Mn, 0.0% Zr and8.2% Co, 1.6% Mn, 1.1% Zr. Fischer-Tropsch testing was as described inExample 1.

The results of F-T testing are illustrated below in Table 3.

Under the conditions tested the Zr oxide promoted catalyst was found toreduce syngas conversion only to a small extent while it had almost noeffect on the product selectivity. Importantly, it decreased the rate ofdeactivation of syngas conversion by a factor of three over the Mnpromoted catalyst. Stability was based on a linear least square estimateof the percent loss in syngas conversion per hour. The increase incatalyst stability observed for the combined Mn and Zr promoted catalystwould further increase both catalyst life and the economic benefit ofthe Mn only promoted catalyst described in Example I. Product qualitywas only slightly changed, showing a partial loss of the increasedolefin content in the product caused by the Mn oxide promoter.

                  TABLE 3                                                         ______________________________________                                        Comparison of Mn and Mn/Zr Oxide Promoted Co TC-123                           Fischer-Tropsch Catalysts                                                     Catalyst:   Co/Mn/Zr/TC-123                                                                              CO/Mn/TC-123                                       ______________________________________                                        Temperature 240° C.                                                                        260° C.                                                                           240° C.                                                                      260° C.                           H2:CO       1:1     1.5:1      1:1   1.5:1                                    Pressure    300     500        300   500                                      Conversion  41      77         48    81                                       CH4         3.5     10.0       3.3   12.2                                     C2-C4       7.7     11.0       7.8   10.4                                     C5-350° F.                                                                         21.4    29.0       21.4  29.6                                     350-650° F.                                                                        29.7    27.9       29.0  30.1                                     650° F.+                                                                           37.7    22.0       38.6  17.7                                     C5+         88.8    79.0       88.9  77.4                                     C4 ole./par.                                                                              2.6     1.0        3.7   .9                                       Stability.sup.(a)                                                                         --      .007       --    .02                                      ______________________________________                                         Space velocity = 300 GHSV                                                     .sup.(a) Percent loss in syngas conversion per hour.                     

EXAMPLE IV

The following example compares a TC-103 and a γalumina supported cobaltFischer-Tropsch catalyst promoted with manganese oxide. The TC-103supported catalyst showed superior product selectivity over theγ-alumina catalyst and in addition demonstrated much superior stability.

The two catalysts were prepared under identical conditions using themethod described for the catalysts in Example II. In the aluminasupported case Kaiser γalumina was substituted for the molecular sievesupport. The calculated percent cobalt and manganese in the catalyst,based on the raw materials used, was in both cases 8.2% Co and 1.6% Mn.Fischer-Tropsch testing was as described in Example I, with bothcatalysts exposed to 220° C., 300 psig, 1:1 H2:CO syngas, and 300 GHSVafter hydrogen treatment.

Results of testing are illustrated in Table 4 below.

The major advantage of using TC-103 as the support was shown in C5+product selectivity and particularly in stability of the catalyst underthe conditions tested. The γ-alumina catalyst showed a 0.03% syngasconversion loss per hour compared to the TC-103 catalyst which showedessentially no deactivation over the period tested. The rate ofdeactivation was based on a linear least squares analysis of the loss insyngas conversion over the length of the test. The analysis included allexcept the first data point for each catalyst, Table 4 shows only thefirst and last data point used for the analysis.

The activity and selectivity of the two catalysts were fairly similarwith the γ-alumina catalyst showing slightly higher initial activity.However, under practical conditions and realistic expected catalyst lifetimes the activity of the γ-Al₂ O₃ supported catalyst would be grosslyinferior relative to the TC-103 supported catalyst. In addition, theTC-103 catalyst showed slightly better product selectivity: reducedmethane, higher C5+.

                  TABLE 4                                                         ______________________________________                                        Comparison of Mn Oxide Promoted γ-Alumina and TC-103                    Cobalt Fischer-Tropsch Catalysts                                              Catalyst:   Co/Mn    γ-Al2O3                                                                           Co/Mn/TC-103                                   ______________________________________                                        HOS         186      258        338   403                                     HOS at 260° C.                                                                     48       120        48    168                                     Conversion  60.7     58.2       58.1  57.6                                    CH4         8.8      10.0       7.6   8.6                                     C2-C4       12.1     13.6       10.3  10.6                                    C5-420° F.                                                                         42.2     39.0       36.6  36.7                                    420-700° F.                                                                        26.8     29.1       26.8  27.2                                    700° F.+                                                                           10.1     8.3        18.7  16.9                                    C5+         79.1     76.5       82.1  80.8                                    C4 ole./par.                                                                              2.0      1.9        1.8   1.7                                     Stability.sup.(a)                                                                         .03            .001                                               ______________________________________                                         Conditions: 260° C., 300 psig, 1:1 H2:CO, 300 GHSV HOS = Hours on      stream                                                                        .sup.(a) Percent loss in syngas conversion per hour.                     

While the invention has been described with respect to various specificexamples and embodiments, it is to be understood that the invention isnot limited thereto and that it can be variously practiced within thescope of the following claims.

We Claim:
 1. A cobalt Fischer Tropsch catalyst supported by anultrahydrophobic molecular sieve in combination with an effective amountof a promoter selected from the group comprising Mn oxide and thecombination of Mn oxide and Zr oxide.
 2. A cobalt Fischer-Tropschcatalyst according to claim 1 wherein the promoter is Mn oxide.
 3. Acobalt Fischer Tropsch catalyst according to claim 1 wherein thepromoter is a combination of Mn oxide and Zr oxide.
 4. A cobalt FischerTropsch catalyst according to claim 1 wherein the ultrahydrophobicmolecular sieve support is a acid extracted LZ-10 molecular sieve.
 5. Acobalt Fischer Tropsch catalyst according to claim 1 wherein theultrahydrophobic molecular sieve support is a steam treated and acidextracted LZ-210 molecular sieve.